Behavioral effects of chronic undernutrition were investigated. Subjects were 138 children, age 6-8, in rural Gautemala who had participated in the INCAP Longitudinal Study. Pre- and postnatal intakes of calorie supplements were the independent variables. Dependent measures were assessments of interpersonal behavior, affect, and coping with environmental requirements. These measures were obtained by observing children in small group activities with peers, and from individual cognitive tests. The results indicate the importance of adequate energy intake in infancy for later behavioral functioning. High calorie supplementation from birth to 2 years was related to higher levels of responsivity, involvement with the environment, and emotional expressivity as well as to moderate activity level, and low levels of passivity. Except for attentional measures, cognitive measures, were not strongly predicted by supplement intake.